This invention relates to the manufacture of resins that are particularly useful in the paper and wood industry, and in particular to stabilized polyamidoamine-epichlorohydrin resins, aqueous solutions thereof, and use in products such as paper products.
Wet strength resins are often added to paper and paperboard at the time of manufacture. In the absence of wet strength resins, paper normally retains only 3% to 5% of its strength after being wetted with water. However, paper made with wet strength resin generally retains at least 10% to 50% of its strength when wet. Wet strength is useful in a wide variety of paper applications, such as toweling, milk and juice cartons, paper bags, and liner board for corrugated containers. Wet strength resins can also provide increased dry strength to paper.
Polyamidoamine-epichlorohydrin resins are also used as creping adhesives. In the manufacture of some paper products such as facial tissue, bathroom tissue, or paper towels, the paper web is conventionally subjected to a creping process in order to give it textural characteristics such as softness and bulk. The creping process typically involves adhering a web (a cellulose web in the case of paper) to a rotating creping cylinder, e.g., the apparatus known as a Yankee dryer, and then dislodging the adhered web with a doctor blade. The impact of the web against the doctor blade ruptures some of the fiber-to-fiber bonds within the web and causes the web to wrinkle or pucker.
Polyamidoamine-epichlorohydrin (PAE) resins are widely used as wet strength resins. These resins can be manufactured by the reaction of polyamidoamine with epichlorohydrin under basic conditions. The resulting resin is then contacted with an acid to stabilize the product. However, the acid also hydrolyzes unreacted epichlorohydrin to species such as 1,3-dichloropropanol (1,3-DCP), 2,3-dichloropropanol (2,3-DCP), and 3-chloropropanediol (CPD). Other chlorine-containing contaminants can also be present.
These epichlorohydrin hydrolysis products are also known as “epichlorohydrin byproducts” or “organic halogenated byproducts.” Present commercial polyamidoamine-epichlorohydrin resins can contain 0.5 to 10 wt % (on a dry basis) of 1,3-DCP, 2,3-DCP, and CPD.
Environmental pressures to produce resins with lower levels of absorbable organic halogen (AOX) species have been increasing. AOX species include epihalohydrin and its hydrolysis products, for example, 1,3-DCP, 2,3-DCP, and CPD, as well as organic halogen bound to the polymer backbone. Production of resins with reduced levels of epihalohydrin and its byproducts has been the subject of much investigation, as described for example, in U.S. Pat. Nos. 7,081,512; 5,256,727; and 5,972,691. Polyamidoamine-epichlorohydrin wet strength resins have limited shelf life and stabilization is critical for providing effective performance properties over storage time. Techniques to reduce chlorinated monomer by-products have been shown to negatively impact storage stability, and this effect is more pronounced for resin products manufactured at high resin solids. Stabilization of PAE resins has also received much attention, as described for example, in U.S. Patent No. However, there is a continuing need for methods of making polyamidoamine-epihalohydrin resins that have low levels of epihalohydrin and its hydrolysis products, while also providing improved storage stability at higher resin solids.